1. Field of the Invention
The present invention relates generally to modulators and amplifiers used in communication and reproduction of audio signals. More particularly, the present invention relates to amplification and modulation equipment where a carrier is modulated in at least one sideband in parametric sound reproduction.
2. Related Art
Modulation of a carrier signal to incorporate audio signal information is well known. Single side band (SSB) and double side band (DSB or AM) modulation including “upper” and “lower” sidebands have been used in radio frequency (RF) communication equipment for many decades to transmit and reproduce audio information. More recently, in the field of parametric audio reproduction, modulation of an ultrasonic carrier signal has been performed. The signal is amplified and fed trough an ultrasonic transducer to produce an audible reproduction of the audio information.
Typically, at least one sideband is used to carry audio signal information. Audio information can be reproduced in a parametric array comprising a fluid medium wherein the transducer is located. The fluid is typically air, but can be other fluids, such as water, for example. The array medium is excited by the transducer at the modulated carrier frequency; and, typically, by non-linear interaction of molecules of the air (or water) medium, audible audio waves are produced. Those audible waves reproduce the audio information in the modulated carrier signal. It will be appreciated that other signal processing can be done, but toward that subject, adherence to the subject matter at hand requires forbearance in setting out more.
Parametric sound reproduction has numerous potential applications. The relatively large power requirements of sound reproduction using this technique is an issue recognized across a range of these applications. Inherently, this technique requires more power than direct excitation of the medium at an audible frequency (audio frequency). For example, conventional audio systems directly generate compression waves reproducing audio information. But in parametric reproduction, the compression waves are created at a higher frequency than that of the audio signal, typically 10 or more times the frequency. Thus changing the excursion direction of the transducer driven element and the coupled medium takes place typically at least this many more times, each change consuming energy. Development of efficient techniques for modulation and amplification of a signal to be sent to the transducer (speaker) as a modulated carrier signal can be of significant benefit in parametric sound reproduction applications. This is because the process of parametric sound reproduction is so inherently power-hungry, improvements in power efficiencies go directly to the bottom line of better reproduction, lower power consumption, and enabling more volume at the distances from the transducer within the array that are of interest in the particular application, especially where that distance is large and/or the desired sound pressure level (SPL) is large at the distance of interest.
Furthermore, the processes of modulation of the carrier, and amplification of the audio source signal, can themselves introduce distortion. For example, audible artifacts of switching in an output stage of a switching amplifier can be problematic. Further, in switching amplification and modulation at frequencies close to the carrier frequency, e.g. about ten times the frequency or less, very noticeable and distracting artifacts are present using conventional techniques. These distortions/artifacts can be noticeable, and distracting, when heard by a listener in a parametric array. They can generally degrade the quality of the audio information heard. Much work has been done in attempting to reduce such undesirable artifacts and distortion. However, due to the necessity for relatively high signal strength, and for increased power in the transducer, these problems remain apparent, and can degrade the listening experience of the hearer in the parametric array.
Improvements in power efficiency and audible signal quality will go far to increase acceptance of parametric sound reproduction technologies. They can be of benefit in other areas where modulation of a reference or carrier signal is used, as well. However, it will be understood that for sake of clarity and understanding, the invention will be set forth in the application by means of example. Most of the discussion of the details of implementation will be relevant to parametric sound reproduction using an ultrasonic carrier. But the improvements disclosed herein may well have important applications in other areas.